According to the tax invoice, the observatory is based upon a standard Shed City shed measuring 2.7 m x 2.83 m x 1.95 m. I thought this size was just about perfect and will hopefully one day comfortably house a 12-inch astrograph on a heavy duty mount (budget pending). I suspect one could fit a 14-inch SCT with room to spare, but I wouldn't push my luck with a larger model.
http://www.flickr.com/photos/mlparkinson/sets/